Redundantly actuated mobile robots using conventional wheels need a precise coordination of their actuators in order to guarantee a safe and precise motion without generating high internal forces and slippage. Using the instantaneous centre of rotation (ICR) of the chassis to describe this motion is a well established method. But the ICR is a mathematical concept which is hardly achieved on a real robot. This paper addresses the problem of ICR estimation of a non-holonomic omnidirectional mobile robot using conventional wheels. Instead of estimating the ICR in the working space, our approach estimates it in the actuators' space. The algorithm is presented in its general form and then adapted for a particular robot. The use of the algorithm with other omnidirectional robots is also discussed. Results from extensive testing done in simulation as well as with a real robot are presented, demonstrating the effectiveness of the proposed method.
AZIMUT-3 is a nonholonomic omnidirectional platform design using sidewards off-centred compliant wheels. This design makes it possible to experiment with the use of the chassis' instantaneous centre of rotation (ICR) for motion control. Research on ICR-based motion controllers has focused on handling structural singularities and misses a more general consideration of the chassis' kinematic and physical constraints like steering, velocity and acceleration constraints. This paper presents the design of an ICR-based motion controller for AZIMUT-3. Leveraging a new parametrization of the motion state space and the associated representation in R 3 (collectively referred to as the H representation) and adapting a time scaling principle initially developed for manipulator trajectories, the designed motion controller is able to handle actuators coordination and their physical limits, as well as structural singularities. Results of tests done with the platform are presented, demonstrating the applicability of the proposed motion controller in efficiently handling these issues.
In order to move safely and accurately, mobile platforms using steerable wheels require adequate coordination of their actuators. One possibility to achieve actuator coordination is to control the motion of the chassis' instantaneous centre of rotation (ICR) and motion around it. Considering the chassis as a rigid body, the ICR is located at the intersection of each wheel's zero motion axis. In practice however, these axes may not concur, in particular when compliant actuators are used for wheel steering. They then no more define precisely an ICR and only an estimation of its position can be computed. Moreover, most parametrizations of the ICR position bring in singularities with no physical meaning, which hinder estimation. This paper introduces the H representation, a new parametrization of the motion state space free of any non-structural singularities, and presents an algorithm which estimates the ICR within the joint space. The proposed approach is compared in terms of reliability, efficiency, accuracy and robustness with three methods working within the operational space. Results suggest that the proposed estimation approach provides the best compromise for these performance indicators.
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